Scores are defined such that 100 is always the current average. But you can compare results to the actual questions today from a decade ago, and see what IQ people today would have gotten back then. This tells you how much IQ has drifted.

IQ used to go up a couple points each decade (known as the Flynn Effect), but this result shows that in more recent times it has been going down.

From Wikipedia on the Flynn Effect: "When intelligence quotient (IQ) tests are initially standardized using a sample of test-takers, by convention the average of the test results is set to 100 and their standard deviation is set to 15 or 16 IQ points. When IQ tests are revised, they are again standardized using a new sample of test-takers, usually born more recently than the first. Again, the average result is set to 100. However, when the new test subjects take the older tests, in almost every case their average scores are significantly above 100."

I believe the posted study is discussing the reason why this trend of increasing scores might no longer hold.

The article doesn't address that, but I have to assume they're talking about non-normalized scores.